The invention relates to an illumination device and its application to a display device. It is especially applicable to polarisation splitting, to focusing and to spectral splitting of a light beam as well as enlargement of a beam by anamorphosis.
Especially, it finds one particular application in display using a monochromatic and trichromatic liquid-crystal screen.
In order to generate large-size video images, the current tendency is to use liquid-crystal active matrices in projection devices.
The projection of coloured images can be achieved either from 3 monochromatic valves or from a single valve fitted with colour filters.
These image-projection techniques, which are appealing for their compactness if they are compared to cathode-ray tubes, have the drawback of having a poor light yield of the order of a few percent. The principal limiting causes being the following:
The electrooptic effect in the liquid crystal requires working with polarised light and leads to a loss of 50% of the light emitted by the source to which it is necessary to add the absorption of the polarisers. PA1 The useful surface of each pixel is reduced because of the space occupied by the transistor and the control electrodes. This factor constitutes the principal limitation for devices using small-size and high-resolution cells (spacing of the pixels &lt;100 .mu.m) which will be required for HDTV needs. PA1 Projection solutions using a single valve have the advantage of being implemented in particularly simple devices. On the other hand, they lead to a loss of luminosity by a factor of at least three for each of the three chromatic components, a loss imposed by the spatial distribution of the colour filters. Since the non-negligible absorption of these filters is incompatible with the use of intense light sources, projection devices having one valve are therefore currently limited to the projection of small-size colour images. PA1 The 16/9 format of HDTV, very poorly suited to the emission diagram of the light sources, leads to the search for architectures which include beam-anamorphosis functions. PA1 as is illustrated in FIGS. 8a and 8b, angular .DELTA..THETA..sub.o =7.5.degree. (in air) and spectral .DELTA..lambda.=40 nm pass-bands are obtained in structures, of approximately 10 .mu.m thickness (d) and having an index variation .DELTA.n of approximately 0.03, operating in reflection mode (HR type) or operating in transmission mode (HT type). They are consequently compatible with trichromatic sources of the arc-lamp type (.DELTA..lambda..sub.i =10 nm; the typical diameter of the 3 mm sources collimated with the aid of a 30 mm focal-length condenser and opened to f/1 corresponds to an angular divergence .DELTA..THETA..sub.o =6.degree.). PA1 The polarising holographic functions have the same angular and spectral pass-band properties. PA1 The "slanted" holographic functions (grating inclined at an angle .phi. in the thickness) enable the chromatic dispersion to be limited (.delta..THETA..sub..lambda. .apprxeq.1 mrd/nm).
This is why the invention relates to an illumination device characterised in that it comprises a holographic polarisation-splitting device illuminated by a non-polarised light source and a holographic focusing device receiving a polarised beam from the holographic polarisation-splitting device and enabling this beam to be focused.
Such an illumination source is applicable to the display of a liquid-crystal device which requires a polarised light.